13 de junio de 2021

Coexistence and adaptive shifts in gazelles

Gazelles are antilopin bovids belonging to the genera Gazella, Eudorcas, Nanger, Litocranius, Ammodorcas, Antidorcas, Antilope and Procapra. In general, only one species of gazelle occurs in any given habitat. Where coexistence is achieved, this tends to be by means of regional shifts in body-size and -shape which correspond partly to differentiation into subspecies.

The best-known example is in the Serengeti ecosystem. Thomson's gazelle (Eudorcas thomsoni nasalis) and Grant's gazelle (Nanger granti granti) coexist here by means of divergent body masses, adults of the former subspecies weighing less than half the average for the latter subspecies in the case of females, and about a third in the case of males. The local form of Thomson's gazelle is the smallest of its genus while the local form of Grant's gazelle is the largest of its species. There is also divergence in water-dependence and movement patterns, with the larger form sedentary and the smaller form migratory and dependent on drinking.

The dorcas gazelle (Gazella dorcas) and the dama gazelle (Nanger dama) coexisted until recently on the southern and western fringes of the Sahara. This was achieved mainly by means of an extreme difference in body sizes and the maximum height of foraging. The former species is the smallest gazelle In Africa while the latter is the largest of all antilopin bovids. In Arabia, the particularly diminutive local form of the dorcas gazelle (Gazella saudiya) has been hunted to extinction while the Arabian sand gazelle (Gazella marica) has survived. These forms differ little in body sizes, suggesting that the Saudi gazelle was more restricted in substrate type and vegetation type than was the case in North Africa, limiting its population even before persecution by humans.

Bearing these patterns in mind, it is interesting to consider how coexistence has been achieved among certain poorly-known gazelles on the Horn of Africa. The gerenuk (Litocranius walleri) coexists as a large northern subspecies with Soemmerring's gazelle (Nanger soemmerringi) and as a small southern subspecies with Grant's gazelle (see https://www.inaturalist.org/observations/44453752). Notwithstanding the greater specialisation of the gerenuk for bipedal foraging, it is puzzling that, in e.g. Tsavo National Park, Peter's subspecies of Grant's gazelle (Nanger granti petersi) has similar body mass (a possible average of 30-35 kg for adult females) to the local form of the gerenuk. A similar puzzle, at larger body sizes, may apply to Soemmerring's gazelle and the gerenuk in Somaliland, a destination for adventurous naturalists.

Anotado en junio 13, domingo 19:09 por milewski milewski | 0 comentarios | Deja un comentario

Peters' gazelle differs from Grant's gazelle in ways resembling genus Gazella

Peters' gazelle (Nanger granti petersi) occurs only in eastern Kenya, and differs from the other subspecies in several ways apart from the shape of the horns in the male.

The body mass of adult males is probably only about 50 kg (similar to that of the male impala, Aepyceros melampus), compared to 60 kg or more in nominate Nanger granti granti. Adult females seem not to have been weighed, but my guess is 35kg or less, compared with an average of 40 kg recorded for the nominate subspecies.

Peters' gazelle is the only subspecies lacking a lateral bleeze - i.e. a pattern on the flanks which is conspicuous enough in profile that it makes the figure stand out rather than blending into the environment - in any individual. This is because Peters' gazelle has only a faint form of the dark streak on the posterior part of the flank (above the stifle-fold), typical of juveniles in the other subspecies and particularly accentuated in populations of Nanger granti notata living on the Laikipia plateau in Kenya.

All forms of Nanger granti, at all ages and of both sexes, have a conspicuous whitish pattern - which I call a posteriolateral bleeze - on the buttocks and extending slightly on to the rump. In Peters' gazelle, the extension on to the rump is minimal and divided by a fawn mid-line, thus resembling the genus Gazella rather than any of the other forms of Nanger. Furthermore, white hardly extends on to the tail-stalk in Peters' gazelle, leaving the tail mainly dark; and photos suggest that the tail is slightly longer, proportional to body size, than in the other subspecies of Nanger granti.

In the facial colouration, the dark rostral spot is minimal in all individuals of Peters' gazelle. And another detail is that the whitish on the front of the upper hindleg is minimal in Peters' gazelle.

Peters' gazelle is the form of genus Nanger most closely resembling Gazella in colouration, and can be regarded as a replacement for Gazella beyond the southernmost limit of that genus on the Horn of Africa. However, it remains larger than any form of Gazella; it emphasises the pygal band (i.e. the dark band running vertically on the posterior of the haunch, adjacent to the whitish buttock) and de-emphasises the dark flank-band to a degree not seen in any species of Gazella; and the tail has a slimmer tassel and is less demonstrative in its movements than in Gazella.

Anotado en junio 13, domingo 00:02 por milewski milewski | 0 comentarios | Deja un comentario

10 de junio de 2021

Beginning a study of the malar stripe in bovids

Many species of bovids, in the antilopin, aepycerotin, caprin and hippotragin tribes, have a feature of colouration called the malar stripe. This is a relatively dark stripe running from the orbit above the eye, diagonally across the side of the face, to near the corner of the mouth. The malar stripe is particularly typical of gazelles, but it also occurs in goats and oryxes, among others.

The adaptive function is deeply ambivalent. On one hand, running through the eye suggests a disruptive effect reducing the conspicuousness of the animal to predators. The eye tends to be one of the most noticeable parts of an animal, and disguising it can be disproportionately effective in helping to hide the whole animal. On the other hand, where the stripe is bold enough, and offset by strikingly pale fur, it can contribute to a flag or even a bleeze, advertising the animal for social purposes. These contrary functions could occur within a given species, according to age or sex.

Here I illustrate a few examples of how subtle the effects remain, even once the dichotomy is understood between reducing conspicuousness and boosting conspicuousness.

In gazelles, the malar stripe tends in most species to be effacing. However, in Soemmerring's gazelle (Nanger soemmerringi, see https://www.inaturalist.org/photos/45006655) it has been drafted to form one border of a bold black-and-white pattern on the front of the face. This species accordingly possesses a facial flag or even a frontal bleeze. In the case of the springbok (Antidorcas marsupialis), which has a showily whitish face, the malar stripe seems perverse. It is not bold enough to offset the pale when viewed at distance, and by crossing out the eye seems to detract from the facial advertisement (see https://www.inaturalist.org/observations/56238889).

The malar stripe of the gemsbok (Oryx gazella) is part of a conspicuous pattern of dark and pale on the face of the adult (see https://www.inaturalist.org/observations/74839633). (In the same species, it plausibly disguises the face shortly after birth when the infant relies on lying low; see the fourth photo in https://fossilrim.org/animals/gemsbok). However, the scimitar-horned oryx (Oryx dammah, see https://www.inaturalist.org/photos/122472718) is analogous to the springbok in being, overall, a species with conspicuously pale colouration. And, as in the case of the springbok, the malar stripe seems to detract from the showiness of the face, which leaves us puzzled as to its adaptive value.

Anotado en junio 10, jueves 23:42 por milewski milewski | 0 comentarios | Deja un comentario

08 de junio de 2021

The peculiar caudal flag of the western dama gazelle

The western dama gazelle (Nanger dama mhorr) has such striking colouration that it is easy to overlook how peculiar the tail is relative to other gazelles.

All species of Gazella, Eudorcas and Ammodorcas have a tail that is large, dark and mobile enough to constitute a caudal flag, particularly in juveniles. The tail is wagged and/or raised in various situations, postures and gaits. For example, the erect tail of the goitred gazelle (Gazella subgutturosa) is so obvious when the animal flees that an alternative name for the species is 'black-tailed gazelle'. In Thomson's gazelle (Eudorcas thomsoni) the tail is wagged nervously as the alert or initially alarmed animal walks slowly and indecisively about in the face of possible danger.

The tail skeleton of the western dama gazelle is about as long, proportional to the body, as in other gazelles, but the tassel - proportionately the smallest of any gazelle - looks negligible. Furthermore, the tail is not demonstrative in adults, being largely inert except when swished to shoo insects. Even when the adult stots to show off its fitness, the tail is kept down. One way to interpret this is that the whole figure is so conspicuously emblazened, with the rump and buttocks in particular so conspicuously white, that accentuation by the tail is superfluous in this subspecies.

The tail of the infant of the western dama gazelle is small relative to those of the infants of other gazelles, owing to the extreme diminution of the tassel. However, it does constitute a caudal flag (see https://www.alamy.com/gazelle-in-blijdorp-rotterdam-the-netherlands-image240604149.html). At birth, the tail is pale fawn at the base and tip, but covered in white erectile hairs along the middle of the stalk. The tail is raised and wagged demonstratively when the infant stots playfully, as well as while it suckles. Until the horn-tips emerge, the infant has a thoroughly cryptic and camouflaged pattern of colouration except for the raised or moving tail.

What is peculiar about the western dama gazelle is not that the tail is most conspicuous in infancy and when waved. Instead it is that this conspicuousness is achieved by means of only a limited tract of fur, which is brought to some contrast with the dullness of the rest of the figure by means of piloerection. The result is that, in this subspecies, the tail is converted from the most noticeable part of the infantile figure to one of the least noticeable parts of the adult figure (see https://www.alamy.com/gazelle-in-blijdorp-rotterdam-the-netherlands-image240604268.html) - with minimal anatomical change as the animal grows. More than in any other antilopin bovid, the western dama gazelle simply outgrows its tail.

Anotado en junio 08, martes 03:51 por milewski milewski | 0 comentarios | Deja un comentario

07 de junio de 2021

How does infantile colouration inform subspecies in the dama gazelle?

Any given species of ungulate usually consists of subspecies, living in different parts of the geographical distribution. Subspecies usually differ somewhat in adult colouration, but many naturalists would be surprised to find that the colouration of infants varies accordingly. Newborns can hardly be expected to be subspecifically distinct in colouration given that they rely mainly on lying so low that they are out of sight of both conspecifics and potential predators. Furthermore, there is a notion that 'ontogeny recapitulates phylogeny', suggesting that infants would tend to retain an ancestral pattern of colouration.

Variation within the dama gazelle (Nanger dama) was clinal over a vast, contiguous distribution, any boundaries between subspecies being blurred. This has led to a recommendation in iNaturalist that no attempt be made to identify photos below species-level. Who, then, would have predicted that the infants of the westernmost and easternmost forms of the dama gazelle are easily distinguished by their colouration?

Owing to captive breeding, we now have clear photos of several individual infants of each form, and these show that the western and eastern dama gazelle differ in colouration already at birth. This is all the more surprising because the infantile colouration of the western dama gazelle is so different from the adult colouration that, viewed in isolation, it would not be identified as the same species, let alone subspecies.

In the western dama gazelle, the conspicuous white markings of the adult are absent in the infant, developing only after the horn-tips erupt (see https://www.alamyimages.fr/photo-image-bebe-gazelle-dama-dama-mhorr-nanger-92733857.html and https://www.zooborns.com/zooborns/2017/03/endangered-dama-gazelles-arrive-with-the-spring.html and https://www.biolib.cz/en/image/id304238/) . By contrast, in the eastern dama gazelle the hornless infant is already relatively pale overall with white emerging on the face and lower flanks (see https://www.zooborns.com/zooborns/2008/11/baby-gazelle-at-the-national-zoo.html and https://www.washingtonian.com/2017/09/21/prepare-to-fall-in-love-with-the-new-baby-gazelles-at-the-national-zoo/ and https://www.smithsonianmag.com/smithsonian-institution/dama-gazelle-calf-born-smithsonian-national-zoo-180970587/).

While these surprising differences need not, in themselves, validate the subspecies of the dama gazelle, they show the cline to be more profoundly differentiated than previously assumed. Should our scientific names not reflect this differentiation? Given that the populations are now artificially separated (partly because the species is virtually extinct in the wild), my recommendation is to resume the use of the subspecies names mhorr (western), dama (central) and ruficollis (eastern), while acknowledging that these subspecies originally graded into each other.

Anotado en junio 07, lunes 07:39 por milewski milewski | 0 comentarios | Deja un comentario

02 de junio de 2021

The eclipsed life of the western dama gazelle

The western dama gazelle (Nanger dama mhorr), exterminated in the wild half a century ago, is photogenic and unmistakable in colouration. However, several aspects of its biology and conservation are easily overlooked even if the naturalist is familiar with the many hundreds of photos taken in zoos and breeding centres.

All existing photos of this subspecies show the descendants of just one male, which was captured together with three females in Western Sahara. Rescuing the subspecies by means of captive breeding with such a limited founder-population was all the more precarious because reproductive rate of this species may depend on numbers owing to a gregarious and polygynous social system. Perhaps this limitation helps to explain why it has proven difficult to reintroduce the dama gazelle to reserves in Africa.

The full sexual dimorphism of the western dama gazelle is hidden in all current populations by the intense management. Adolescent males are allowed to breed in captivity and reintroduction, but this may be a suboptimal mating system. Masculine brawn tends to keep growing for years after sexual maturity, as revealed by a few photos of three lonely males in Hadj National Wildlife Refuge, before the last of that reintroduced population was killed by corrupt Tunisians, wiping out millions of dollars of investment. One photo in particular, in https://www.cokesmithphototravel.com/expedition-to-tunisia.html, shows how proportionately small the head and horns of the mature male can become, suggesting a fully mature body mass almost double that of the adult female. No other species of antilopin bovid is so dimorphic in this way, consistent with a breeding system which would tend to falter in small, remnant populations.

Although its ability to forage with free-standing bipedality might suggest a social system similar to that of the barely-gregarious gerenuk (Litocranius walleri), the dama gazelle is more like the springbok (Antidorcas marsupialis) in its migratory hypergregariousness. Even if a reintroduced population is protected in some small reserve in Senegal or elsewhere in its original range, it cannot be truly rewilded unless numbers and movements are restored, which is unlikely. Therefore the subspecies seems condemned to functional extinction as a wild animal - even if it eventually succeeds enough on Texan ranches to be commercially hunted there.

The distinctive and consistent colouration of adults, including fully mature males, hardly prepares the naturalist for the appearance of the infants. There is no subspecies of gazelle or other antilopin bovid in which the ontogenetic change in pattern is so great. In all gazelles, the infant has plainer colouration than the adult, but in the western dama gazelle the difference is extreme, as revealed by several clear photos of newborns in zoos (e.g. https://www.biolib.cz/en/image/id304240/ and https://www.alamyimages.fr/photo-image-bebe-gazelle-dama-dama-mhorr-nanger-92733857.html and https://www.zooborns.com/zooborns/2017/03/endangered-dama-gazelles-arrive-with-the-spring.html). Can the social biology of the western dama gazelle explain why infants have distinctive colouration whereas fully mature males do not?

Anotado en junio 02, miércoles 08:32 por milewski milewski | 0 comentarios | Deja un comentario

01 de junio de 2021

Regional interspecies mimicry in conspicuous colouration of ruminants?

All three species of the largest ruminants in the Saharan region have extremely pallid colouration, unmatched by desert ungulates elsewhere. Why have the scimitar-horned oryx (Oryx dammah), addax (Addax nasomaculatus, https://en.wikipedia.org/wiki/Addax#/media/File:Addax_nasomaculatus.jpg) and dama gazelle (Nanger dama, https://treshabarger.com/2018/05/15/dama-gazelle-2/) converged in this way? Likewise, four sympatric species of ruminants in western North America resemble each other in pale patterns, in this case a bold whitish patch on the hindquarters. Why have the wapiti (Cervus canadensis), mule deer (Odocoileus hemionus), bighorn sheep (Ovis canadensis) and pronghorn (Antilocapra americana) converged in this way? In both cases, the regional convergences have arisen independently in unrelated clades, as if to suggest some sort of interspecies mimicry.

In Muellerian mimicry, various toxic or venomous species converge on a single pattern of warning colouration, apparently to reinforce a shared message to potential predators. In the case of the above ungulates, there is no question of warning colouration in this strict sense, because there is scant ability to harm predators in self-defence. Instead the relevant anti-predator tactic, in addition to gregarious vigilance and fleeing, is of self-advertisement of individual fitness and vigour. This show-off pattern, accentuated by stotting gaits, persuades the scanning predator that the individual has no debility that would make it vulnerable. Could something similar to Muellerian mimicry apply here?

The relevant message to the potential predator is 'before you attack me, here is some honest information suggesting that it may be too costly to you'. In the case of Muellerian mimicry as strictly defined, the information is about chemical harm that the prey animal could inflict on the predator. But if we broaden the concept, then information about fitness could be similarly useful inasmuch as a futile chase costs the predator energy and opportunity while at the same time risking accidental injury and the attracting of unwanted attention from the predator's own predators.

If this working hypothesis makes enough sense to deserve testing, which new name should we adopt for this type of interspecies resemblance?

Anotado en junio 01, martes 01:55 por milewski milewski | 0 comentarios | Deja un comentario

29 de mayo de 2021

Surprising variation in infantile colouration in gazelles

All gazelles and other antilopin bovids hide their newborns for at least one week, during which the infant lies as low as possible. Because these antelopes live in relatively open vegetation, concealment of infants depends partly on inconspicuous colouration.

In the gerenuk (Litocranius walleri), the gazelle most likely to hide by means of inconspicuous colouration, it is unsurprising that the infant has virtually identical colouration to the adult. However, those species which have conspicuous colouration in adults vary surprisingly in the patterns of infants. Take, for example, Thomson's gazelle (Eudorcas thomsoni, see https://www.shadowsofafrica.com/thomson-s-gazelle-rufifrons) and the western dama gazelle (Nanger dama mhorr, see https://www.cbd-habitat.com/en/2019/07/02/the-first-reintroduction-project-for-mhorr-gazelle-into-the-wild/). The former has a striking contrast between the dark flank-band and tail and the adjacent pale surfaces, while the latter has conspicuous white on the rump, lower flanks, upper legs and face.

The infant of Thomson's gazelle (see https://www.alamy.com/stock-photo-newborn-baby-grants-gazelle-79192809.html and https://www.flickr.com/photos/davidbygott/7842769892) has colouration similar to that of the adult, the conspicuous markings being suppressed to some extent by posture, body-proportions and the relative length of the fur. By contrast, the infant of the western dama gazelle (see https://www.biolib.cz/en/image/id304240/) is so different that, were it observed in isolation, one could scarcely tell which species - or even genus - it belongs to.

The adults of neither species make much attempt to hide. What differs is that the conspicuous features are already present at birth in Thomson's gazelle whereas they are not in the western dama gazelle. Particularly odd is that the infant of the western dama gazelle lacks most of the pale on the hindquarters, in contrast to Thomson's and other gazelles, in which erectile white fur on the buttocks is precocial. In both species, the tail is most conspicuous in the infant, and frequently erected in excitement. However, the difference is that in the infant of the western dama gazelle the only conspicuous feature on the whole figure is the white mid-section of the tail. White becomes conspicuous on its buttocks only when the infant grows into a juvenile.

Anotado en mayo 29, sábado 06:45 por milewski milewski | 0 comentarios | Deja un comentario

27 de mayo de 2021

The odd masculinity of the dama gazelle

Please consider three trends among species of ungulates. The larger the body of the species, the more massive the mature male tends to be relative to the adult female; the larger his horns tend to be relative to his body size; and the more conspicuous he tends to be in colouration, relative to the female. Another way of putting this: the larger the mature male is relative to the female, the more 'outsize' he seems to be in bulk, head-adornments, and showy colouration.

Because these overall trends of scaling make sense in terms of gregariousness, polygyny, and masculine rivalry, it is the exceptions that are more intriguing than the species conforming to the trends. And the dama gazelle (Nanger dama) is one such exception.

The dama gazelle has the male up to twice as massive, in full maturity, as the adult female. This is not particularly odd, because it is the largest species of antilopin bovid, a clade in which most species have the male only modestly larger than the female and the smallest species predictably have the male no larger than the female.

What is odd is that the horns of the fully mature male dama gazelle (e.g. see https://www.cokesmithphototravel.com/expedition-to-tunisia.html) are the smallest, not the largest, relative to body size among gazelles. And that his colouration, while certainly showy, is no more showy than that of the female.

Compare this with the blackbuck (Antilope cervicapra). This species is only about average in body size for a gazelle, and the male is not much larger than the female. Yet the mature male has extremely large horns while the female lacks horns altogether; and his colouration is so much showier than hers that they look like different species.

This suggests that the showy pale/dark pattern of colouration shared by the dama gazelle and the blackbuck, in which white extends so high on the figure that it catches the light and makes the animal stand out even at a distance, has arisen for different reasons in the two species. In the dama gazelle, conspicuousness is a 'female-centred' adaptation facilitating social cohesion in general, and the male conforms to this even by keeping his horns small. By contrast in the blackbuck, conspicuousness is a 'male-centred' adaptation which tends to undermine the hiding of the inconspicuous females and juveniles from predators. In the dama gazelle, males presumably compete mainly by contests of brawn, whereas in the blackbuck males compete mainly by contests of attire and ritual fencing.

Anotado en mayo 27, jueves 23:40 por milewski milewski | 0 comentarios | Deja un comentario

23 de mayo de 2021

Interpreting the buccal semet of the pronghorn

The pronghorn (Antilocapra americana) has an odd-looking mouth. It also has an oddly graphic pattern of colouration at the mouth, which I suggest we call a buccal semet in view of its potential function of accentuating movements of the mouth as part of social communication within the species.

The mouth of the adult male is shown in https://www.featheredphotography.com/blog/2013/08/28/odd-growths-on-the-face-of-a-pronghorn/. That of the adult female is shown in https://www.endangered.org/saving-the-sonoran-pronghorn-will-help-save-us/. And https://www.zooborns.com/.a/6a010535647bf3970b01538fdcdd79970b-popup shows that this is the first feature of adult colouration to develop in the infant.

The fact that there is a crisp dark/pale pattern at the mouth can be explained in terms of the mutual monitoring of cud-chewing among group-members. Many deer and bovids have comparable patterns although the precise design is unique to the pronghorn. Keeping an eye out for any interruption of rumination in companions could signify alarm, because any individual noticing potential danger immediately interrupts the 'white noise' of the chewing to listen up in the appropriate direction.

However, what remains odd about this species is that the pattern at the mouth is so black-and-white and consistent regardless of age, sex, season, and individuality - which affect other features, both large- and small-scale, of the colouration of the pronghorn. Most other ruminants have subtle, not stark, versions of the buccal semet.

A possible explanation lies in a behavioural oddity of the pronghorn, which moves its lips in ways still poorly understood. Lip-smacking is certainly a social signal, given e.g. when the male is about to copulate. However, compulsive lip-trembling is the really odd behaviour. Mueller-Schwarze and Mueller-Schwarze in 1972 wrote: "After sniffing materials such as crushed grass, rotten apples and the cud of deer, pronghorn of both sexes move their lips rhythmically at a rate of 150/minute for a long period, sometimes for longer than 20 minutes. During that period they may show interest in food...but are unable to eat. When put in the mouth the food falls out again".

Why would the pronghorn signify to group-members the discovery of bacterial odours in such an extreme way?

Anotado en mayo 23, domingo 23:46 por milewski milewski | 0 comentarios | Deja un comentario